It is a very small addon witch does nothing else then synching the watched status of different kodi-DBs. Therefore it is not important. whether you have your kodi-DBs in a SQL-DB on a NAS or directly stored on a device.
In the opposit to trakt it needs no internet connection and is much faster and smarter, very reliable and for the home or local use only.

The addon is very smart - it synchs my watched status in only some seconds directly after starting kodi or spcm or whatever. If you have watched an episode or a movie. it synchs your watched-DB or file directly after it.
So I have exact the same watched status on all devices indepenend from database-version.

Beeing a big fan of MrMC what makes my FireTv to a real mediaplayer, it would be really really nice, if you could implement watchedList in MrMC.

I don't trust trakt (reliability) and also they have no APIs for exporting your data (lock-in). I was really missing this plugin, so I created a quick hack to one-way sync data from Mr.MC to a watchedlist compatible format.

It does require you to use MySQL though. It creates an additional db named watchedlist in the Mr.MC database. It can then be dumped into sqlite3 format and used as a db for the WatchedList plugin. I literally just thew it together and I might or might not improve upon it in the future .